If the promise of nanotechnology is to be fulfilled, nanoparticles will have to be able to make something of themselves. An important advance toward this goal has been achieved by researchers who have found a simple and yet powerfully robust way to induce nanoparticles to assemble themselves into complex arrays. By adding specific types of small molecules to mixtures of nanoparticles and polymers, they were able to direct the self-assembly of the nanoparticles into arrays of one, two, and even three dimensions with no chemical modification of either the nanoparticles or the block copolymers. In addition, the application of external stimuli, such as light and/or heat, can be used to further direct the assemblies of nanoparticles for even finer and more complex structural details, a result verified by small-angle x-ray scattering (SAXS) at the ALS.

Plug-and-Play Nanostructures

Nanotechnology's potential for breakthroughs that reverberate through nearly every sector of the economy (medicine, computing, energy, homeland security) is so great that it has already generated new research communities and billions in funding. But precise control of the spatial organization of nanoparticles and other nanoscopic building blocks over multiple length scales has been a bottleneck in the bottom-up generation of technologically important materials. Various routes to direct nanoparticle assemblies have been explored, including the use of DNA and functional polymers. However, large-scale fabrication poses a significant hurdle for many practical applications.

Zhao et al. have demonstrated a simple yet versatile approach to precisely controlling the spatial distribution of readily available nanoparticles over multiple length scales, ranging from the nano to the macro. The beauty of this technique is that it involves no sophisticated chemistry. It really is a plug and play technique, in which you simply mix the nanoparticles with the block copolymers and then add whatever small molecules you need. Bring together the right basic components—nanoparticles, polymers and small molecules—stimulate the mix with a combination of heat, light or some other factors, and these components will assemble into sophisticated structures or patterns. It is not dissimilar from how nature does it.

An electron micrograph shows a self-assembled composite in which nanoparticles of lead sulfide have arranged themselves in a hexagonal grid.

Nano-sized particles—bits of matter a few billionths of a meter in size, or more than a hundred times smaller than the stuff of today's microtechnologies—display highly coveted properties not found in macroscopic materials, including optical, electronic, magnetic, etc. The promise of nanotechnology is that exploiting these unique properties on a commercial scale could yield such "game-changers" as sustainable, clean and cheap energy, and the creation on demand of new materials with properties tailored to meet specific needs. Realizing this promise starts with nanoparticles being able to organize themselves into complex structures and hierarchical patterns, similar to what nature routinely accomplishes with proteins.

Small as they are, nanoparticles are essentially all surface, so any process that modifies the surface of a nanoparticle can profoundly change the properties of that particle. Precisely arranging these nanoparticles is critical to tailoring the macroscopic properties during nanoparticle assembly. Although DNA has been used to induce self-assembly of nanoparticles with a high degree of precision, this approach only works well for organized arrays that are limited in size; it is impractical for large-scale fabrication. Another approach is to use block copolymers—long sequences or "blocks" of one type of monomer molecule bound to blocks of another type of monomer molecule.

Block copolymers readily self-assemble into well-defined arrays of nanostructures over macroscopic distances. They would be an ideal platform for directing the assembly of nanoparticles except that block copolymers and nanoparticles are not particularly compatible with one another from a chemistry standpoint. A "mediator" is required to bring them together.

The researchers found such a mediator in the form of small molecules that will join with nanoparticles and then attach themselves and their nanoparticle partners to the surface of a block copolymer. For this study, the group used two different types of small molecules, surfactants (wetting agents) dubbed "PDP" and "OPAP." These small molecules can be stimulated by light (PDP) or heat (OPAP) to sever their connection to the surface of a block copolymer and be repositioned to another location along the polymeric chain. SAXS studies of the samples, performed at ALS Beamline 7.3.3, confirmed the nanoparticles' changes in position under various conditions. In this manner, the spatial distribution of the small-molecule mediators and their nanoparticle partners can be precisely directed without the need to modify either the nanoparticles or the polymers.

Left: SAXS profile of a blend of PDP and CdSe nanoparticles during a heating and cooling cycle. The profile shows that the blend went through two thermoreversible transitions between three assemblies of CdSe nanoparticles. Right: Schematics depicting how the nanoparticles were rearranged during heating and cooling: the nanoparticles (blue) assembled in the center of PDP layers from 50 to 100°C (bottom), at the interfaces between the layers at 110°C (middle), and were randomly distributed in the layers at 150°C (top).

For this study, PDP or OPAP small molecules were added to various blends of nanoparticles, such as cadmium selenide (CdSe) and lead sulfide (PbS), mixed in with a commercial block copolymer—polystyrene-block-poly (4-vinyl pyridine). While this group worked with light and heat, other stimuli, such as pH, could also be used to reposition small molecules and their nanoparticle partners along block copolymer formations. Strategic substitutions of different types of stimulus-responsive small molecules could serve as a mechanism for structural fine tuning or for incorporating specific functional properties into nanocomposites. The researchers are now in the process of adding functionality to their self-assembly technique.

Research funding: Army Research Office; National Science Foundation; DuPont; 3M; Japan Ministry of Education, Science, Sports, and Culture; and the U.S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.